1. Field of the Invention
The present invention generally relates to a switching power supply circuit including a reference voltage generating circuit capable of preventing inrush current from flowing into the switching power supply circuit and reducing overshoot when the voltage rises to a set voltage.
2. Description of the Related Art
In a switching power supply circuit, there is a technology known to a person skilled in the art to increase a reference voltage for determining an output voltage using a soft-starting circuit so that inrush current is prevented from flowing into the switching power supply circuit when the output voltage rises to a predetermined voltage on the supply of electric power.
FIG. 27 is an example of such a switching power supply circuit according to a related art.
As illustrated in FIG. 27, an error amplifying circuit 242 outputs an error voltage EAo24 obtained by amplifying the difference in voltage between a divided voltage VFB24 obtained by dividing an output voltage Vout24 at a voltage dividing resistor 246 and a reference voltage VREF24 output from a soft-starting circuit 241.
A comparator 244 compares a triangular wave signal TW24 output from a triangular wave generating circuit 243 with the error voltage EAo24, and generates and outputs a switching pulse signal Spa24 for carrying out PWM control to a switch circuit 245.
Note that soft-starting circuits, such as the aforementioned soft-starting circuit 241, are generally configured to include a capacitor and a resistor; however, in a case where the capacitor is installed in a chip, since the size of the capacitor is large, areas of the capacitor may be too large to be installed in the chip. Moreover, in a case where the capacitor is installed outside, since the number of nodes is increased, the number of components will be increased.
Therefore, there are provided some soft-starting circuits that do not use such a capacitor but use a digital signal and a DA converter instead (see Japanese Laid-Open Patent Application No. 2004-23948).
However, as illustrated in FIG. 28, when the reference voltage VREF24 ramps up in a gradual linear manner, overshoot occurs in the output voltage Vout24 which causes the output voltage Vout24 to exceed a set voltage. In such cases, a longer time may be required for stabilizing the output voltage Vout24, or an undesired defect may occur in a load supplied with the output voltage Vout24.
As a technology for reducing overshoot, as illustrated in FIG. 29, there is disclosed a method of applying the reference voltage VREF24 in a gradual changing manner such that the output voltage Vout24 is output in a gradual changing manner. Accordingly, the overshoot can be reduced when ramping up the reference voltage VREF24.
However, large inrush current still flows into a capacitor C241 as shown in FIG. 30 when ramping up the reference voltage VREF24 as shown in FIG. 29. Accordingly, harmonic noises are unfortunately increased as can be shown from a spectrum waveform in FIG. 31.